<asp:TextBox ID="EmailTextBox" runat="server" Text='<%# Bind("Email") %>'
                                    MaxLength="50" />

<asp:RequiredFieldValidator ID="RequiredFieldValidator1" ControlToValidate = "EmailTextBox" Display ="Dynamic" runat="server" ErrorMessage="Please enter email"></asp:RequiredFieldValidator>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate = "EmailTextBox" Display ="Dynamic" ValidationExpression="^w+([-+.']w+)*@domain.com$"
    ErrorMessage="Please enter valid email address. Eg. Something@domain.com"></asp:RegularExpressionValidator>

protected void btnSubmit_Clicked(object sender, EventArgs e)
{
    if (Page.IsValid)
    {
        //Do Submit Stuff.
    }
    else
    {
        //Error handle
    }
}